By the 17th century, Muslims were at peace under the Ottoman Khilafah following the 4 Sunni Madh-habs....
Until....one man appeared in the scorching deserts of central Arabia (Najd).
That man was "shaykh" Muhammad bin Abd-al-Wahhab al-Tamimi (1703-1792).
This man, influenced by the fanatical puritanical ideas of Ibn-Taymiyyah and by the inherent harshness of the desert...started a new Sect.
This Sect was called by him and his followers as: "al-Muslimun", then "Ahl al-Tawhid", "al-Wahhabiyyah", "Ahl al-Hadith "and then finally "al-Salafiyyah".
Muhammad bin Abd-al-Wahhab believed that ALL THE MUSLIMS of his time were actually kafirs and mushriks. In other words, they were all apostates (Murtadds)....including all the Ulama of Makkah and Medina, and even the Islamic Khalifah in Turkey.
He reckoned, by visiting the Tombs of the Prophet (SAW) and the Awliya and seeking their Barakah...muslims had reverted to SHIRK and IDOL-WORSHIP.
He also reckoned that by following the 4 Imams, muslims had abandoned the SUNNAH.
Anyways, he considered everyone but his followers to be Kafir, and therefore declared WAR against the Muslim Khilafah State, forcing everyone to become a Wahhabi-Salafi or be killed.
As soon his evil Fitna started to spread, the Ulama of the Muslims World started to write Fatwas and books refuting his dark takfiri ideology.
For more info, http://www.sunna.info/antiwahabies/wahhabies/htm/spy1.htm